Twisted pair (T-2) electric cables have been in use since the early 1970's, with thousands of miles of cable being installed since that time. The T-2 cable comprises two electrical conductors that are joined in a gradual twist about every 9 feet. The cable is twisted to combat wind induced motion, such as Aeolian vibration, galloping and sub-conductor oscillation, which can damage the cable's insulators and hangers.
Aeolian vibration is caused by low to moderate winds which create vortices around the electric cable, causing it to vibrate up and down. Galloping occurs when an airflow around a cable produces lift forces that cause it to move up and down in a wavelike motion. That is especially problematic when ice builds up on the front of an electric cable, creating an air-foil shape and increasing the wind induced lift forces acting on the cable. Sub-conductor oscillation occurs in non-twisted electric cable pairs where a front conductor takes the brunt force of the wind and casts a wind-shadow on a second conductor trailing the first, causing the second conductor to vibrate in an elliptical motion with respect to the first.
The twist in the T-2 cable resists the wind induced motion by constantly varying the diameter of the electric cable, preventing the build-up of resonant vibration, and producing a low torsional stiffness in the cable.
However, for the electric cable twist to remain constant and effective, it is important that equal tension in the two conductors of the cable be maintained. Uneven tension in the conductors can cause a looping or sagging effect, where one conductor loops around the other, causing a single conductor to bear the weight of both. That increases the likelihood that the electric cable will detach from a power pole, or otherwise makes the cable unstable.
Currently, an even tension is achieved in the T-2 cable by pre-twisting the conductors and binding them together using a metal band. The cable is then pulled through stringing rollers located on power poles to set up the cable for attachment to the power pole. However during this process the tension in the cables may become uneven. That can be caused by having stringing rollers that are too small or having an angle of approach that is too steep. Both these features can cause the roller to “pinch” the cable and push the twist back, thereby unevening the tension in the cable. To correct that problem, the cable is attached to a snatch block that evens the tension in the conductors after being pulled through the stringing rollers.